Production of fuel ethanol has significantly gained in popularity in the last few years. Nowadays more than 80 such plants are in operation in the USA alone. This enthusiasm has been driven by the need for a replacement for MTBE and the desire to become less energy-dependant on other countries. A typical ethanol production plant will use yeast (typically Saccharomyces) to produce an average of 50 millions gallons of pure ethanol per year, mostly but not exclusively from hydrolyzed corn starch in large fermentors of more than 2 millions liters.
The basic carbon and energy sources for yeast growth are sugars. Unmodified starch can not be used because yeast does not contain the appropriate enzymes to hydrolyze this substrate to fermentable sugars. Beet and cane molasses are commonly used as raw material in fermentation because the sugars present in molasses, a mixture of sucrose, fructose and glucose, are readily fermentable. In addition to sugar, yeast also require certain minerals, vitamins and salts for growth. Some of these can be added to the blend of beet and cane molasses prior to flash sterilization while others are fed separately to the fermentation. Alternatively, a separate nutrient feed tank can be used to mix and deliver some of the necessary vitamins and minerals. Required nitrogen is supplied in the form of ammonia and phosphate is supplied in the form of phosphoric acid. Each of these nutrients is fed separately to the fermentation to permit better pH control of the process. The sterilized molasses, commonly referred to as mash or wort, is stored in a separate stainless steel tank. The mash stored in this tank is then used to feed sugar and other nutrients to the appropriate fermentation vessels.
In high volume commercial ethanol production, corn starch is the substrate of choice, because of its low cost and availability. In using corn starch, often a first partial hydrolysis step (using alpha-amylase) precedes a co-saccharification fermentation step (in the presence of glucoamylase) which is then followed by distillation.
A yeast preparation is used to inoculate various fermentors, including so called propagators, usually this occurs at the co-saccharification fermentation stage. The co-saccharification fermentation insures a controlled and progressive hydrolysis of the dextrins produced in the previous partial hydrolysis step. This simultaneous hydrolysis and fermentation provides for a slow release of sugars and insures that the yeast is not exposed to a punctual, very large, osmotic pressure that would exist if all the dextrins had already been hydrolyzed at the beginning of the fermentation and prior to yeast inoculation.
The production of yeast for use in commercial fermentation is, in itself, a multi-step process. In many cases, commercial production requires that the yeast be packaged, stored and shipped in large quantities in a manner that guarantees the purity and viability of the final yeast product.
Baker's yeast production, for example, often starts with a pure culture tube or frozen vial of the appropriate yeast strain. This yeast serves as the inoculum for the pre-pure culture tank, a small pressure vessel where seed is grown in medium under strict sterile conditions. Following growth, the contents of this vessel are transferred to a larger pure culture fermentor where propagation is carried out with some aeration, again under sterile conditions. These early stages are conducted as set-batch fermentations. In set-batch fermentation, all the growth media and nutrients are introduced to the tank prior to inoculation.
From the pure culture vessel, the grown cells are transferred to a series of progressively larger seed and semi-seed fermentors. These later stages are conducted as fed-batch fermentations. During fed-batch fermentation, molasses, phosphoric acid, ammonia and minerals are fed to the yeast at a controlled rate. This rate is designed to feed just enough sugar and nutrients to the yeast to maximize multiplication and prevent the production of alcohol. In addition, these fed-batch fermentations are not completely sterile. It is not economical to use pressurized tanks to guarantee sterility of the large volumes of air required in these fermentors or to achieve sterile conditions during all the transfers through the many pipes, pumps and centrifuges. Extensive cleaning of the equipment, steaming of pipes and tanks, and filtering of the air is practiced to insure as aseptic conditions as possible.
At the end of the semi-seed fermentation, the contents of the vessel are pumped to a series of separators that separate the yeast from the spent molasses. The yeast is then washed with cold water and pumped to a semi-seed yeast storage tank where the yeast cream is held at approximately 34 degrees Fahrenheit until it is used to inoculate the commercial fermentation tanks. These commercial fermentors are the final step in the fermentation process and are often referred to as the final or trade fermentation.
Trade fermentations are carried out in large fermentors with working volumes up to 50,000 gallons. To start the commercial fermentation, a volume of water, referred to as set water, is pumped into the fermentor. Next, in a process referred to as pitching, semi-seed yeast from the storage tank is transferred into the fermentor. Following addition of the seed yeast, aeration, cooling and nutrient additions are started to begin the 15-20 hour fermentation. At the start of the fermentation, the liquid seed yeast and additional water may occupy only about one-third to one-half of the fermentor volume. Constant additions of nutrients during the course of fermentation bring the fermentor to its final volume. The rate of nutrient addition increases throughout the fermentation because more nutrients have to be supplied to support growth of the increasing cell population. The number of yeast cells increase about five- to eight-fold during this fermentation.
Air is provided to the fermentor through a series of perforated tubes located at the bottom of the vessel. The rate of airflow is about one volume of air per fermentor volume per minute. A large amount of heat is generated during yeast growth and cooling is accomplished by internal cooling coils or by pumping the fermentation liquid, also known as broth, through an external heat exchanger. The addition of nutrients and regulation of pH, temperature and airflow are carefully monitored and controlled by computer systems during the entire production process. Throughout the fermentation, the temperature is kept at approximately 86 degrees Fahrenheit and the pH is generally in the range of 4.5-5.5.
At the end of fermentation, the fermentor broth is separated by nozzle-type centrifuges, washed with water and re-centrifuged to yield a yeast cream with a solids concentration of 15 to 24%, and often in the 18% range. The yeast cream is cooled to about 45 degrees Fahrenheit and stored in a separate, refrigerated stainless steel cream tank. Cream yeast can be loaded directly into tanker trucks and delivered to customers equipped with an appropriate cream yeast handling system. Alternatively, the yeast cream can be pumped to a plate and frame filter press and dewatered to a cake-like consistency containing 27-33% yeast solids. This press cake yeast is crumbled into pieces and packed into 50-pound bags that are stacked on a pallet. The yeast heats up during the pressing and packaging operations and the bags of crumbled yeast must be cooled in a refrigerator for a period of time with adequate ventilation and placement of pallets to permit free access to the cooling air. Palletized bags of crumbled yeast are then distributed to customers in refrigerated trucks. Cream yeast can also be further processed into dried yeast (92-97% solids) by using a fluid bed dryer or similar types of dryers.
In contrast, yeast production for fuel ethanol plants is significantly different. Even though fuel ethanol plants are very large, they consume much less yeast than industrial bakeries. A large industrial bakery, for example, will take anywhere between 1 and 4 to 5 truckloads of 20,000 liters of cream yeast (average of 18% solids) per week. It is therefore common to see cream yeast systems installed at those bakeries; they are usually comprised of two large refrigerated, agitated receivers that can receive at least one truckload of liquid yeast each, a distribution ring to the dough mixers and a full cleaning in place (CIP) system. By analogy, a large fuel ethanol plant will typically use 200 to 500 kg of dry yeast per week or the equivalent of 1500 to 2500 liters of cream yeast per week. Such a reduced usage, however, cannot justify installing sophisticated cream yeast systems that are common in the baking industry.
Fuel ethanol plants, in contrast, use dry yeast and a series of propagation tanks to multiply and activate the yeast. The use of such propagation tanks reduces the amount of yeast required and effectively eliminates the need for refrigerated storage. Dry yeast has the additional benefit of having a relatively long shelf life (up to about 3 months). Unfortunately, dry yeast loses part of its fermentative activity during the drying process as well as during rehydration. Moreover, dry yeast is in a dormant state (hence the propagation step commonly seen in fuel ethanol plants) and is not as fast as fresh yeast (27-33% solids).
The ethanol industry has also been using fresh yeast but fresh compressed or crumbled yeast requires refrigeration for storage and has an average storage life is 2 to 3 weeks. Compressed yeast can of course be kept for longer period of time (up to 6 weeks) but this results in a significant loss of activity and also allows for the possibility development of molds on the surface of the yeast. Liquid cream yeast (15 to 24%) suffers the same aging and refrigeration problem and requires agitation because of the natural tendency of yeast to sediment.
None of the above-mentioned forms in which yeast is currently supplied is fully satisfactory. The rather small quantities of yeast required in commercial ethanol facilities, as compared to the size of the operations, does not make it economical to deliver small amount of yeast on a frequent basis to those plants. In many cases, fresh yeast may not be available or the fresh compressed yeast may become dry, moldy or inactive. Conversely, a dry yeast product remains active for a long period of time but, in any case, it must be awakened properly and is not as fast as fresh yeast; this is one of the main reasons why propagators are common in the industry. As to cream yeast, it has the tendency to sediment to the bottom of the container in which it is transported. Consequently the cream yeast has to be stirred before use.
Thus the present invention aims at eliminating the disadvantages described above.